1. Field of the Invention
The invention pertains to a process for preparing the end of a pipe for drawing over a mandrel, and to a rolling mill for performing the method.
2. Description of the Related Art
When pipes of considerable length are drawn with a flying mandrel, cracks often occur during the initial phase of the drawing operation. This is caused by the eccentricity of the pipe, that is, by its irregular wall thickness. Thus, when a dent is made in one side of the pipe so that the mandrel can be carried along, the longitudinal axis of the mandrel may not be exactly horizontal in the pipe, i.e., coaxial with the axis of the pipe. When drawing is started, it is therefore possible for the mandrel to enter the drawing ring at a slant, which causes the pipe to crack at this point.
From the technical literature (Herstellung von Rohren [Production of Pipes], Verlag Stahleisen mbH, Dxc3xcisseldorf, 1975, p. 21, lines 5-12) it is known that rolling processes with slanted rolls are able to improve the wall thickness tolerance and thus also correct the eccentricity of the pipe. The worse the eccentricity of the pipe to be rolled, the greater is the improvement. This is true not only for rolling from the heat of rolling, but also for cold-rolling processes such as the transverse or press-rolling of pipes. Especially effective in this case are three-roll rolling mills, because the arrangement of the three rolls has the effect of uniquely determining the geometry of the rolling process. A tolerance-improving effect begins at an elongation or stretching of the rolling stock of only 10%. The effect increases with the degree of elongation.
DE 43 13 648 discloses an apparatus for the stepwise stretching of pipes. With the help of this device, considerable reductions in wall thickness can be achieved, which are similar to those of the known Pilger cold rolling process.
The object of the invention is to provide a process for improving the initial drawing conditions with a flying mandrel during straight drawing or even drawing with bull blocks can be improved.
This object is achieved by a process for preforming the end of the pipe to be drawn by inserting a preforming mandrel and rolling the pipe against the mandrel to form a constriction.
In the solution according to the invention, instead of a dent pressed into one side of the pipe, a constriction extending all the way around the pipe is introduced, which forces the drawing mandrel into a horizontal position, i.e. coaxial with the axis of the pipe, during the initial drawing phase. In addition, the setup for introducing the constriction is combined with the setup for reducing the eccentricity, and the two different work steps are thus executed in a single pass.
According to the invention, to improve the eccentricity of the end of the pipe to be drawn, a three-roll rolling apparatus with non-driven rolls spaced 120xc2x0 apart is used, the rolling head of which revolves around the pipe. The rolls can be tightened inward in the radial direction during the rolling process, so that the rolls can travel over a specific contour on the surface of the pipe.
A mandrel having a tapered tip and a section with a diameter corresponding to the inside diameter of the pipe is inserted in the pipe. The tip comprises a first section which is substantially smaller than the inside diameter of the pipe and a second or intermediate section which corresponds to the contour of a constriction to be rolled into the pipe. The third section is the section having a diameter corresponding to the inside diameter of the pipe. The second section has a concavely curved surface which tapers from the diameter of the third section the diameter of the first section. The rolls are brought into contact with the outside of the pipe adjacent to the first section of the mandrel. When the rolls are tightened inward as the rolling head rotates and moves axially toward the second section, a circumferential constriction is rolled into the pipe. The rolling head is then moved axially relative to the mandrel, toward the third section, as the head rotates and the rolls move radially outward. The constriction thereby assumes a contour corresponding to the diameter of the second section of the mandrel. As a result of the three-roll arrangement, the eccentricity is improved simultaneously with a reduction in the wall thickness. The head continues to rotate while moving axially with respect to the third section of the mandrel so that the desired reduced thickness is achieved to the end of the pipe. After this rolling step, one or more drawing mandrels are pushed into the end of the pipe, and a drawing device is fixed to the end of the pipe. The drawing mandrel has a tapered end which abuts the constriction and is thereby coaxially aligned in the pipe prior to drawing.
The ratio xcex4 between the unrolled and the rolled cross section of the pipe is determined as follows:
xcex4=(D0/S0)/(D1/S1)xe2x89xa70.75xe2x89xa60.95.
where:
xcex4=the reduction ratio;
D0=the outside diameter of the pipe before rolling;
D1=the outside diameter of the pipe after rolling;
S0=the wall thickness of the pipe before rolling; and
S1=the wall thickness of the pipe after rolling.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.